Ion-Energy Dependency in Proton Irradiation Induced Chemical

Article pubs.acs.org/JPCC

Ion-Energy Dependency in Proton Irradiation Induced Chemical Processes of Poly(dimethylsiloxane) Robert Huszank,*,† Szabolcs Z. Szilasi,† and Dezső Szikra‡ †

Institute for Nuclear Research, Hungarian Academy of Sciences, P.O. Box 51, H-4001 Debrecen, Hungary Department of Physical Chemistry, University of Debrecen, P.O. Box 7, H-4010, Debrecen, Hungary

‡

ABSTRACT: In this paper, we present a study of chemical changes in poly(dimethylsiloxane) (PDMS) induced by proton irradiation of various energy and fluence. It has been found that the forming products vary as the energy of the proton changes, which means that the energy of the incident ions can influence the chemical mechanism. This is probably caused by the variations in the ion−molecule interactions, that is, the change of probability of ionization and excitation of the molecules. We propose reaction mechanisms for the processes taking place in PDMS by different energy proton irradiation. This unique effect may be used for various purposes, such as to create advanced materials with buried ion-induced modifications or to understand better the heavy ion irradiation induced reactions which have crucial importance for example in proton therapy. The chemical changes created in PDMS were characterized by universal attenuated total reflectance infrared spectroscopy (UATR-FTIR).

1. INTRODUCTION It is known that irradiation of materials with ionizing radiation is accompanied by radiation-induced effects, that is, changes of the chemical structure and/or physical properties. The radiolysis of organic and inorganic materials by gamma, Xray, or e-beam irradiations have been investigated for a long time, while the heavy ion radiolysis of materials and the induced chemical processes are less studied, as shown in comprehensive summaries on this topic.1,2 The final products formed during ion-irradiation in a certain material depend on the formation and reactions of reactive intermediates such as excited states, ions, and free radicals. However, it is not known yet how these reactions are affected by the characteristics of the irradiation source (i.e., the particle type, its energy, and linear energy transfer (LET)). Thus, the relation between the ion−molecule interactions (atomic collisions) and the occurred chemical reaction is still not well understood, although it plays a very important role in space, medical, and materials science or in ion beam therapy. Proton beam is known as a powerful tool for the treatment of tumor tissues. Accelerated protons have a dose distribution that makes this method able to localize the radiation dose more precisely during the treatment, so the radiotherapy-induced side effects can be reduced significantly by sparing normal tissues. Because of this advantage, proton therapy has gained increasing attention in the past decade3,4 even though the chemical processes, that take places, are not well-known. Generally, organic materials are sensitive against high-energy ionizing radiations (e.g., accelerated ions, gamma ray, X-ray). The minimum energy required to cleave a covalent bond of a carbon chain is in the range of approximately 3−6 eV. This energy range is easily surpassed by ionizing radiations, © 2013 American Chemical Society

representing high probability of degradation to organic materials. The linear energy transfer is defined as the average energy loss of the particle per unit path length (dE/dx). During ion-irradiation, the energy transfer to the surrounding medium increases along the ion track, according to the Bragg equation, and it has a local maximum value around the end of range region (Bragg peak). This increase in local energy transfer leads to higher concentrations of reactive species, which can affect the radiation chemical yield.5,6 However, the reaction mechanism and the forming products may vary along the ion track as well, which process can hardly be explained by the LET value. Instead, this is probably due to the changes in the way of the ion−molecule interactions, that is, ionization versus excitation of the molecules, which is probably more related to the actual energy of the irradiating ion then to the deposited energy density. Poly(dimethylsiloxane) (PDMS) is a promising polymer, serving as a base material for many applications nowadays.7,8 Its chemical structure is based on a silicon main chain (−Si−O− Si−) with methyl side groups (−CH3). The siloxane bonds have some special features, which strongly influence the chemistry of the polysiloxanes. The Si−O bond distance is shorter than the sum of the covalent radii, 1.64 Å, instead of 1.76 Å, which suggests a partial double bond character of the Si−O bond. Furthermore, the barrier of the rotation around the Si−O axis, as well as the linearization of the Si−O−Si angle, is very low.9 Consequently, the chain is unusually flexible; the Si− O−Si angle is between 140°−180°, which is significantly wider Received: July 15, 2013 Revised: November 7, 2013 Published: November 21, 2013 25884

dx.doi.org/10.1021/jp406984d | J. Phys. Chem. C 2013, 117, 25884−25889

The Journal of Physical Chemistry C

Article

than the tetrahedral angle. In addition, there is a significant difference in the bond energies and electronic characters between the main chain and the side groups, making this polymer ideal to test the qualitative chemical changes in the function of the energy of the irradiating ion. The aim of this work is the detailed investigation of the chemical effect of the incident ion energy of the irradiating protons on a model polymer (PDMS). We focused on the characterization of the chemical changes caused by different energy proton irradiations and the explanation of the effect of the proton energy on the chemical mechanism. We have demonstrated that the forming products can also vary in the function of the proton energy. This unique effect can be important in materials science; furthermore, it may be useful for various purposes, such as to create advanced materials where the ion-induced modifications are buried or to understand better the ion-irradiation induced reactions, which have crucial importance for example in proton therapy. The chemical changes created in PDMS are characterized by universal attenuated total reflectance infrared spectroscopy (UATRFTIR).

ion energy on the surface of the samples. To compute the ion ranges and energies in PDMS the SRIM software package was used which concerns the stopping power and the range of ions in matter.13,14 2.3. Universal Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (UATR-FTIR). The infrared spectroscopic measurements were carried out subsequent to the irradiation and outside of the high vacuum chamber with a diamond head Perkin-Elmer Spectrum One type universal attenuated total reflectance Fourier transform infrared spectrometer (UATR-FTIR) equipped with a DTGS detector (4 cm−1 resolution). Equal amounts of force were applied to all samples during the measurements. Analyzing depth is around ∼6 μm. The data were collected and analyzed using Spectrum ES 5.0 software. The concentration of methylene groups was determined by a calibration curve using the dilution of the original uncured PDMS polymer liquid. For the quantitative determination of the hydroxyl groups, hydroxy terminated poly(dimethylsiloxane) was used as standard material, obtained from Aldrich (viscosity 90−150 cSt).

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The PDMS polymer samples were made by the commonly used Sylgard 184 elastomer kit from Dow Corning.10 The base polymer is vinyl-terminated poly(dimethylsiloxane) (average Mn ∼ 125 000, viscosity ∼ 4500 cSt), and the curing agent (vulcanizer) contains platinum catalyst, dimethyl-methyl-hydrogen-siloxane cross-linker, and a tetramethyl-tetravinyl-cyclotretrasiloxane inhibitor. The liquid silicone prepolymer and the curing agent were thoroughly mixed in the recommended ratio of 10:1 and placed in ultrasonic bath for 5 min to remove the formed bubbles. Then the prepolymer was poured in a Petri dish and cured for 30 min at a temperature of 125 °C and allowed to finish curing overnight at room temperature. During the curing process, a hydrosilylation reaction takes place in the presence of the platinum catalyst, resulting in cross-linking of the polymer chains. After curing, the PDMS polymer substrate (about 2 mm thick) was cut to 3 cm diameter discs and these discs were extracted in chloroform (CH3Cl) solvent twice to remove the unreacted, un-cross-linked monomers and curing agent from the network. The samples were dried in ambient condition for 48 h before irradiation. The density of the cured PDMS was measured with a pycnometer, and it was found to be 0.9836 g cm−3. 2.2. H+ ion Irradiation. The H+ irradiations have been performed in an ion irradiation chamber, described in detail elsewhere.11 The energetic ions are produced by a 5 MV single ended Van de Graaff accelerator.12 The ion beam was made homogeneous using a collimator system and a 0.51 μm thin Ni foil; the applied spotsize was 5 mm in diameter. The irradiations were done in an ultrahigh vacuum environment, the beam current was 20 nA. The proton fluence was 1.25 × 1015 ions/cm2 at the ion energy dependent experiments and was varied between 6.25 × 1013−3.13 × 1015 ions/cm2 in the 2.0 MeV irradiation experiments. In the ion energy dependent experiments we were varying the proton energy between 56 keV to 2.00 MeV in small steps and examining the surface in all experiments. To achieve low energy protons from the accelerator, 9 μm thick aluminum foil absorbers were used. The initial energies and absorber thicknesses were determined in order to achieve the desired

3. RESULTS 3.1. Chemical Changes by Different Energy Proton Irradiation. Under ion beam irradiation, complex chemical processes take place in organic materials. For initial steps, the ionization and excitation of the target material occurs as the penetrating ions lose their kinetic energy via inelastic collisions. The excited-state molecules may return to the ground state through radiationless decay or form free radicals by homolytic dissociation reactions. Finally, the free radicals cause a number of chemical reactions in polymers because of their high reactivity. The PDMS polymer was irradiated with different energy protons (56 keV to 2.00 MeV) but with the same fluence (1.25 × 1015 ions/cm2). The infrared spectra of the unirradiated and the irradiated PDMS polymer with different energy protons are shown in Figure 1. The initial PDMS polymer has a series of characteristic IR bands; for the recognized bands see Table 1 and Figure 1 (black line). The IR reference spectrum of the cross-linked PDMS is in accordance with previous published data.15,16 The most intense bands are the −CH3 rocking and −Si−C− stretching (790 cm−1), asymmetric Si−O−Si stretches (1060

Figure 1. FTIR spectra of PDMS polymer irradiated with different energy protons. 25885



Article

Table 1. Assignment of IR Spectra of the Initial and Irradiated PDMS Samples Shown in Figures 1 and 2 IR bands (cm−1)

description

∼3400 2960, 2904 2928, 2876 1720 1625, 1357 1407 1455 1258 1060 900 790

−OH stretching in −Si−OH and −C−OH asymmetric −CH3 stretching in −Si−CH3 −CH2− stretching in −Si−CH2− −CO stretching −CC− stretching, trans alkene asymmetric −CH3 deformation in −Si−CH3 C−H bending symmetric −CH3 deformation in −Si−CH3 asymmetric −Si−O−Si− stretching in [−(CH3)2Si−O−] −Si−O stretching in −Si−OH −CH3 rocking and −Si−C− stretching in −Si−CH3

Scheme 1. Reaction Mechanism Taking Place in PDMS Polymer Irradiated with Lower Energy Protons

cm−1), symmetric −CH3 deformations (1258 cm−1), asymmetric Si−CH3 stretches (2960 cm−1), and Si−OH stretches (∼3400 cm−1). The intensity of the absorption bands of the methyl (2960 cm−1) and the Si−CH3 (1258 cm−1) groups decreased significantly, similarly to the case of the irradiation experiment at high proton energy (see section 3.2). New high intensity broad bands appeared at the 3200−3500 and 1625 cm−1 regions, and several less intensive bands around 2928, 2876, and 1720 cm−1. The broad absorption peak around 3400 cm−1 indicates the formation of hydroxyl groups, for which bands appear only when the energy of the irradiating protons was below a certain level. The mechanism of the formation of hydroxyl groups in PDMS in vacuum environment initiated by heavy ion irradiation was described earlier in detail.11 That study showed that the −OH groups can be formed only if the main −Si−O− Si− chain breaks. Since the irradiation took place in high vacuum environment, without molecular oxygen in the system, the oxygen atom in the −OH group can be originated from the main chain only (no long-lived radicals form during the irradiation in PDMS which could react with molecular oxygen after the irradiation). The band around 1720 cm−1 indicates the presence of carbonyl groups as it was reported earlier as well.11,17 This band appears also only at low ion energy irradiation, similarly to the −OH groups. The mechanism of carbonyl group formation in the presence of molecular oxygen was thought to the reaction of molecular oxygen with a methylene group, which previously formed by cross-linking of methyl groups. However, without molecular oxygen, this mechanism also cannot be realized in high vacuum environment. The formation of carbonyl groups is the result of the recombination reaction of two previously formed radicals, such as oxygen radical and a methylene radical, followed by a bond rearrangement.11 In the consideration of the infrared spectra and the earlier studies with other irradiation experiments, the formations of hydroxyl and carbonyl groups are the dominant processes that take place during the irradiation of PDMS. Furthermore, these processes and the leaving methyl groups lead to the formation of an inorganic, silica-like product (SiOx), which was observed earlier in other studies.17,18 Scheme 1 shows the suggested mechanism of the main reaction that occurs in the PDMS at lower proton energies (taking into account the mechanisms already proposed earlier11,19). 3.2. Chemical Changes by 2.0 MeV Proton Irradiations. In these experiments, the PDMS samples were

irradiated with constant energy protons (2.0 MeV), but with various fluence. Figure 2 shows the infrared spectra of the

Figure 2. FTIR spectra of PDMS polymer irradiated with 2.0 MeV energy protons with different proton fluences.

unirradiated and the irradiated PDMS samples. The intensity of the absorption bands of the methyl (2960 cm−1) and the Si− CH3 (1258 cm−1) groups decreased significantly and two new peaks appeared at 2876 and 2928 cm−1, which belong to the methylene group. The significant decrease of the bands of the −CH3 and the −Si−CH3 groups with increasing ion fluence suggests the direct and indirect degradation of the methyl groups, via cross-linking reaction mechanism and the release of small molecular weight fragments (H2, CH4).20 The appeared bands of methylene group at 2876 and 2928 cm−1 confirmed that cross-linking reaction took place in the samples. We followed the change of the −CH3 group in the function of the proton fluence. The concentration of the methyl groups was determined by a calibration method described in the Experimental Section. Figure 3 shows the concentration of −CH3 groups of the PDMS as a function of the fluence of 2.0 MeV protons. The concentration of the methyl groups decreased significantly during the irradiation, by about 75%. In conclusion, the irradiation of the PDMS with high proton energy causes cleavage in the methyl and the −Si−CH3 bonds, gas yielding, and cross-linking in the polymer. The main −Si− O−Si− chain remained unaffected, even if the fluence of protons was more than two times higher in these experiments, 25886



Article

higher than the bonding energies of Si−C and C−H, which are about 3.3−4.5 eV.23 However, weak fluorescence was observed from the singlet states of many alkanes during irradiation, but even the most strong fluorescence yield was only about 1%. It means that the vast majority of the excited or ionized molecules will release their energy through a chemical reaction, that is, bond cleavage, instead of simple thermalization process.24,25 This mechanism even further favored because of the very limited charge or excited state transfer ability from single covalent bonds. So, if the excitation or ionization occurs on the methyl group by the 2.0 MeV protons, it will undergo a degradation process through the cleavage of C−H or Si−C bonds. The bond energy in Si−O is about ∼4.8 eV, which is considerably higher than the C−H or Si−C bond energies, but still less than the energy deposited by the collisions of energetic protons. Chain scission, followed by hydroxyl group formation was still not observable at this proton energy. It is probably due to the special electronic character of the siloxane bond. These features are accepted as a consequence of the strongly ionic character of the Si−O bond and of the negative hyperconjugation (p(O) → σ*(Si−Y))x, (where Y is any atom bond to silicon) which also causes an elongation of the CH3−Si σbond by adding electron density to its antibonding orbital.26 The interaction is stronger when Y is a more electronegative atom, for example, in our case, carbon. Consequently, when a fast proton ionizes the siloxane chain, the hyperconjugative effect will stabilize the formed cation until a recombination occurs. This prevents the chain scission and the formation of hydroxyl groups. When PDMS is irradiated with lower energy protons (or when the 2.0 MeV protons decelerate in the bulk PDMS), the energy transferred to the medium during an interaction decreases, while the probability of the interactions increases. The formation of hydroxyl groups starts when the energy of the protons is about 0.5−0.6 MeV. At this point, the energy transferred to the molecule through far collisions is not enough for ionization anymore so in the collisions the electronic excitation becomes the most probable energy transfer process. In case of methyl groups, the excitations lead to bond cleavage in 99%, so there is no ion energy dependency in the chemical mechanism. This is in good agreement with observations that, in polymers, where only C−C and C−H bonds exists, the proportion of the chemical changes follows the Bragg curve;27 since all types of the ion-irradiation induced interactions lead to bond scission. In the case of the Si−O−Si bonds, when the main energy transfer process becomes electronic excitation, the high probability of the direct excitations to antibonding orbitals leads to the cleavage of the Si−O bond, which may be further facilitated by the transfer ability of the excited states through the hyperconjugation. Furthermore, at low proton energies, the probability of cross-recombination of the ions and electrons released from another atom is high (because of the high concentration of the electrons and ions in the track), in which the formation of triplet excited states are favored.28 The energy release from a triplet state may also cause the cleavage of the Si−O bonds. In conclusion, during ion−matter interactions (ionic collisions), the energy of the incident ions can strongly affect the chemical mechanism in case of certain type of materials (i.e., PDMS in our case). This kind of ion energy related effect to the chemical mechanism has not been observed yet.

Figure 3. Concentration of −CH3 groups of the PDMS as a function of the fluence of 2.0 MeV protons.

which was proved by the lack of the hydroxyl groups (for detailed explanation, see the next section). Scheme 2 shows the mechanism of the main degradation taking place in PDMS polymer with high energy proton irradiation (2.0 MeV H+). Scheme 2. Mechanism of the Degradation Taking Place in PDMS Polymer Irradiated with High Energy Protons (2.0 MeV H+)

3.3. Effects of the Energy Transfer to the Chemical Changes. The mechanisms of ion stopping in matters are divided into nuclear and electronic collisions. At higher energies and with light incident ions (i.e., H+, He+) the electronic collision dominates.13,21 In organic materials or in macromolecules, the chemically most relevant processes are the ionization and excitation of target atoms and molecules. Higher energy collisions (e.g, the energy transfer to inner shell electrons) play a less important role in the chemical changes, since their cross section is very low. In our recent study, when irradiating PDMS with 2.0 MeV protons, the most probable energy loss process is the far collision. The energy loss in such a collision is about 10−20 eV,22 which is enough to cause ionization since this energy is higher than the first ionization potential of most elements. In the ionization processes, most of the electrons stay in the socalled critical radius, in which recombination and electron excitation may happen after the thermalization of the electrons. In case of the alkanes, that is, the methyl side groups on the PDMS main chain, the energies of the first singlet excited states (LUMO) are about 6.5−7 eV, the triplets are about 5.5−6 eV, which are all antibonding orbitals, while the ionization potentials are about 10 eV. These energies are significantly 25887



Article

during an interaction decreases and the most probable energy transfer process in the collisions turns to be the electron excitation, instead of ionization. Consequently, the electron excitation to antibonding orbitals and the formation of triplet excited states from cross-recombination lead to the cleavage of the Si−O bond, which may be further facilitated by the transfer ability of the excited states through the hyperconjugation. We proposed reaction mechanisms for the reactions taking place in PDMS. Moreover, this unique behavior can give possibility for some interesting applications, such as initiating buried chemical reactions, creating buried optical waveguides, or understanding better the heavy ion irradiation induced reactions, which have crucial importance for example in proton therapy.

The energy loss profile and the energy of the protons during stopping of 2.0 MeV H+ ions in PDMS calculated by SRIM code13,14 is shown in Figure 4a. The penetrating ions stop

■

AUTHOR INFORMATION

Corresponding Author

*Mailing Address: Institute for Nuclear Research, Hungarian Academy of Sciences, H-4026 Debrecen, Bem tér 18/c. Hungary. E-mail: [email protected]. Phone: +36 (52) 509200 x11394. Fax: +36 (52) 416181. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Figure 4. (a) Averaged energy loss profile (black) and the actual energy of the protons (red) during the stopping of 2.0 MeV H+ ions in PDMS, calculated by SRIM code.13,14 (b) Concentration of the formed hydroxyl groups as a function of the penetration depth of 2.0 MeV protons.

Notes

around the depth of 87 μm in the polymer. The energy deposition profile is ascending until it reaches the maximum at about 82 μm depth (Bragg peak), and after that it decreases quickly until reaching zero (where the ions stop). As it was shown, the ion irradiation induced chemical processes in PDMS are strongly affected by the energy transferred to the molecules in one collision. This behavior can give possibility for some interesting application, like initiating buried chemical reactions or creating buried optical waveguides, etc. Figure 4b. shows the concentration of the formed −OH groups as a function of the penetration depth of 2.0 MeV protons in PDMS. One can see that −OH groups were formed around the Bragg peak of the 2.0 MeV H+ only, when the penetrating protons energy is below a certain level.

■

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The technical assistance of the Van de Graaff accelerator operating staff at ATOMKI is gratefully acknowledged. REFERENCES

(1) Tabata, Y.; Ito, Y.; Tagawa, S., Eds. Handbook of Radiation Chemistry; CRC Press: Boca Raton, FL, 1991. (2) LaVerne, J. A.; Chang, Z.; Araos, M. S. Heavy Ion Radiolysis of Organic Materials. Radiat. Phys. Chem. 2001, 60, 253−257. (3) Widesott, L.; Amichetti, M.; Schwarz, M. Proton Therapy in Lung Cancer: Clinical Outcomes and Technical Issues. A Systematic Review. Radiother. Oncol. 2008, 86, 154−164. (4) Olsen, D. R.; Bruland, O. S.; Frekholm, G.; Norderhaug, I. N. Proton Therapy − A Systematic Review of Clinical Effectiveness. Radiother. Oncol. 2007, 83, 123−132. (5) Schnabel, W.; Klaumünzer, S. Linear Energy-transfer Effects in the Radiolysis of Polymers: 2. Main-chain Degradation of Polymethacrylonitrile and Crosslinking of Polystyrene. Radiat. Phys. Chem. 1989, 33, 323−328. (6) LaVerne, J. A.; Baidak, A. Track Effects in the Radiolysis of Aromatic Liquids. Radiat. Phys. Chem. 2012, 81, 1287−1290. (7) Giri, R.; Sureshkumar, M. S.; Naskar, K.; Bharadwaj, Y. K.; Sarma, K. S. S.; Sabharwal, S.; Nando, G. B. Electron Beam Irradiation of LLDPE and PDMS Rubber Blends: Studies on the Physicomechanical Properties. Adv. Polym. Technol. 2008, 27, 98−107. (8) Satriano, C.; Conte, E.; Marletta, G. Surface Chemical Structure and Cell Adhesion Onto Ion Beam Modified Polysiloxane. Langmuir 2001, 17, 2243−2250. (9) Grigoras, S. In Computational Modeling of Polymers; Bicerano, J., Ed.; Marcel Dekker: New York, 1993; p 161. (10) DowCorning Corp. “Product information: Sylgard 184 Silicone elastomer” Form 10-898F-01. (11) Huszank, R.; Szikra, D.; Simon, A.; Szilasi, S. Z.; Nagy, I. P. 4He+ Ion Beam Irradiation Induced Modification of Poly(dimethylsiloxane). Characterization by Infrared Spectroscopy and Ion Beam Analytical Techniques. Langmuir 2011, 27, 3842−3848.

4. CONCLUSIONS In this study, the proton irradiation induced chemical changes of poly(dimethylsiloxane) have been investigated in order to reveal how the incident ion energy influence the reaction mechanism. By the ATR-FTIR measurements, it was found that by varying the energy of the incident ions the forming products vary as well. This means that the energy of the incident ions have a strong effect on the chemical mechanism. Our results indicate that this effect is because of the energy dependent changes of the ion−molecule interactions, that is, ionization versus excitation of the molecules. We discovered that when an energetic proton ionizes the siloxane chain, the hyperconjugative effect will stabilize the formed cation until a recombination occurs, so at high incident energies the Si−O chain remains intact. On the other hand, when 2.0 MeV protons decelerate, the energy transferred to the medium 25888



Article

(12) Borbély-Kiss, I.; Koltay, E.; László, S.; Szabó, Gy.; Zolnai, L. Experimental and Theoretical Calibration of a PIXE Setup for K and L X-rays. Nucl. Instrum. Methods Phys. Res., Sect. B 1985, 12, 496−504. (13) Ziegler, J. F.; Biersack, J. P.; Littmark, U. The Stopping and Ranges of Ions in Solids,; Pergamon Press: New York, 1985; new ed. in 2003. (14) Ziegler, J. F.; Ziegler, M. D.; Biersack, J. P. SRIM - The Stopping and Range of Ions in Matter. Nucl. Instrum. Methods Phys. Res., Sect. B 2010, 268, 1818−1823. (15) Graubner, V. M.; Nuyken, O.; Lippert, T.; Wokaun, A.; Lazare, S.; Servant, L. Local Chemical Transformations in Poly(dimethylsiloxane) by Irradiation with 248 and 266 nm. Appl. Surf. Sci. 2006, 252, 4781−4785. (16) Efimenko, K.; Wallace, W. E.; Genzer, J. Surface Modification of Sylgard-184 Poly(dimethyl siloxane) Networks by Ultraviolet and Ultraviolet/ozone Treatment. J. Colloid Interface Sci. 2002, 254, 306− 315. (17) Waddell, E. A.; Shreeves, S.; Carrell, H.; Perry, C.; Reid, B. A.; McKeel, J. Surface Modification of Sylgard 184 Polydimethylsiloxane by 254 nm Excimer Radiation and Characterization by Contact Angle Goniometry, Infrared Spectroscopy, Atomic Force and Scanning Electron Microscopy. Appl. Surf. Sci. 2008, 254, 5314−5318. (18) Hillborg, H.; Ankner, J. F.; Gedde, U. W.; Smith, G. D.; Yasuda, H. K.; Wikstrom, K. Crosslinked Polydimethylsiloxane Exposed to Oxygen Plasma Studied by Neutron Reflectometry and Other Surface Specific Techniques. Polymer 2000, 41, 6851−6863. (19) Charlesby, A. In Irradiation Effects on Polymers; Glegg, D. W., Collyer, A. A., Eds.; Elsevier Applied Science: London, 1991; pp 39− 78. (20) Hill, D. J. T.; Whittaker, A. K. Radiation Chemistry of Polymers, in Encyclopedia of Polymer Science and Technology; Wiley: New York, 2005. (21) Rousseau, C. C.; Chu, W. K.; Powers, D. Calculations of Stopping Cross Sections for 0.8- to 2.0-MeV Alpha Particles. Phys. Rev. A 1970, 4, 1066. (22) Hummel, A. Radiation Chemistry. The Chemical Effects of Ionizing Radiation and Their Applications. Interfaculty Reactor Institute; Delft University of Technology: Delft, 1995. (23) Rothman, W.; Hirayama, F.; Lipsky, S. Fluorescence of Saturated Hydrocarbons. III. Effect of Molecular Structure. J. Chem. Phys. 1973, 58, 1300−1317. (24) Dellonte, S.; Flamigni, L.; Barigelletti, F.; Wojnarovits, L.; Orlandi, G. Temperature Dependence of the Fluorescence Lifetimes of Linear Alkanes: A Correlation with Photodecomposition. J. Phys. Chem. 1984, 88, 58−61. (25) Ausloos, P.; Rebbert, R. E.; Shwarz, F. P.; Lias, S. G. Pulse- and Gamma Ray- Radiolysis of Cyclohexane: Ion Recombination Mechanisms. Radiat. Phys. Chem. 1983, 21, 27−43. (26) Sheldrick, W. S. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989; Vol. 1, p 242. (27) Szilasi, S. Z.; Huszank, R.; Szikra, D.; Váczi, T.; Rajta, I.; Nagy, I. Chemical Changes in PMMA as a Function of Depth Due to Proton Beam Irradiation. Mater. Chem. Phys. 2011, 130, 702−707. (28) Mayer, J. Ed. Properties and Reactions of Radiation Induced Transients. Selected Topics; Polish Scientific Publisher: Warszawa, 1999.

25889


Ion-Energy Dependency in Proton Irradiation Induced Chemical

Recommend Documents